Yuga (
Devanāgari: युग) in
Hindu philosophy is the name of an epoch or era within a four age cycle. According to
Hindu cosmology, life in the universe is created and destroyed once every 4.1 to 8.2 billion years,
[1][2] which is one full day (day and night) for
Brahma. The lifetime of a Brahma himself may be between 40 billion and 311 trillion years.
[1]The cycles are said to repeat like the seasons, waxing and waning within a greater time-cycle of the creation and destruction of the universe. Like Summer, Spring, Winter and Autumn, each yuga involves stages or gradual changes which the earth and the consciousness of mankind goes through as a whole. A complete yuga cycle from a high
Golden Age, called the
Satya Yuga to a Dark Age,
Kali Yuga and back again is said to be caused by the solar system's motion around another star.
[3]
According to the
Laws of Manu, one of the earliest known texts describing the yugas, the length is 4800 years + 3600 years + 2400 years + 1200 years for a total of 12,000 years for one arc, or 24,000 years to complete the cycle (one precession of the equinox). There is no mention of a year of the demigods or any years longer than the solar, which is consistent with description in The Holy Science.
[3] However, the one debatable interpretation from the Srimad Bhagavatam states the following: "The duration of the Satya millennium equals 4,800 years of the years of the
demigods; the duration of the Dvāpara millennium equals 2,400 years; and that of the Kali millennium is 1,200 years of the demigods... As aforementioned, one year of the demigods is equal to 360 years of the human beings. The duration of the Satya-yuga is therefore 4,800 x 360, or 1,728,000 years. The duration of the Tretā-yuga is 3,600 x 360, or 1,296,000 years. The duration of the Dvāpara-yuga is 2,400 x 360, or 864,000 years. And the last, the Kali-yuga, is 1,200 x 360, or 432,000 years in total." (Śrīmad Bhāgavatam 3.11.19)
[2]. These 4 yugas follow a timeline ratio of (4:3:2:1).
The ages see a gradual decline of dharma, wisdom, knowledge, intellectual capability, life span, emotional and physical strength.